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Description The Multi-Filter Rotating Shadowband Radiometer (Model MFR-7) is a field instrument that simultaneously measures global, diffuse, and direct normal components of spectral solar irradiance. The MFR-7 uses independent interference filter-photodiode combinations, mounted in a temperature-controlled enclosure, to detect spectral irradiance at six wavelengths and in one broadband channel. An automated, microprocessor-controlled shadowband is used to alternately shade and then expose the entrance aperture of the instrument, allowing for the measurement of the three solar radiation components. The global and diffuse components are measured directly and the direct-normal component is computed from the difference of the two measured components. |
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The MFR-7 spectral measurements are made at six wavelengths: 415, 500, 615, 673, 870, and 940 nm. The choice of wavelengths allows the reconstruction of the incident solar spectral irradiance and the determination of optical depths of water vapor, aerosols, and ozone (using the Chappuis ozone absorption bands). The broadband channel measures the total solar irradiance, similar to a pyranometer such as our Model TSP-700. Custom wavelength channel configurations are available on special order. The MFR-7's unique ability to make simultaneous spectral measurements of the three solar irradiance components makes it an extremely versatile instrument. For example, in order to obtain the data collected in the broadband channel of the MFR-7, at least two instruments would have to be used: a total solar pyranometer, and either a tracker-mounted normal incidence pyrheliometer or a second, shaded total solar pyranometer. When making spectral measurements, the MFR-7 has the advantage of sensing the global and diffuse irradiance components with the same detector. This eliminates the error introduced into the measurement of solar irradiance components by the use of different detectors to measure the different components. The MFR-7 consists of two basic components: a Detector Assembly and an Electronics Enclosure. Principle of Measurement and Specification The MFR-7 uses an automated rotating shadowband to make measurements of the global and diffuse components of solar irradiance. Once these two components are known, a CPU can readily compute the direct-normal component. The geometry of the rotating shadowband instrument is shown in the figure below. The shadowband is a strip of metal formed into a circular arc and mounted along a celestial meridian with the instrument's entrance aperture at the center of the arc. The shadowband blocks a strip of sky with a 3.3° umbral angle, sufficient to block the sun. It can be positioned with an accuracy of 0.4° by the microprocessor-controlled stepper motor. The motor housing is adjusted for the latitude of the instrument. When the instrument is installed at the field site, it must be azimuthally aligned to the Earth's pole (North or South, depending on the hemisphere). Once aligned, no further mechanical adjustment is necessary and the instrument may operate for extended periods of time without any operator intervention, unlike trackers that require nearly constant attention. Spectral Response:
10 nm FWHM centered on broadband channel Copyright
CSIRO 2000 ©
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